1. Field of the Invention
The present invention relates generally to flexible media data storage devices and systems, and more particularly, to a head positioning system for adjusting for misalignment between a read/write head and a flexible data storage media, such as magnetic tape.
2. Background Information
Business, science and entertainment applications depend upon computers to process and record data, often with large volumes of data being stored or transferred to nonvolatile storage media, such as magnetic discs, magnetic tape cartridges, or optical disk cartridges. Digital data-recording on magnetic data storage tape remains a viable solution far storage of large amounts of data.
Increased data storage capacity and data storage and retrieval performance is desired of all commercially viable mass storage devices and media. In the case of linear magnetic data storage tape, a popular trend is toward multi-channel movable head structures with narrowed track widths so that many linear data tracks may be achieved on a recording medium of a predetermined width, such as one-half inch width magnetic tape. To increase the storage density for a given cartridge size the bits of data on the recording medium must be accurately written to smaller areas and on the plurality of parallel linear data tracks.
As more data tracks are recorded on a tape, each track becomes increasingly narrow and thus, more susceptible to errors that may be caused by misalignment of the tape head to the data tracks. One known problem, as a result of the increase in track density, is tape skew with respect to a reference centerline of the tape head.
The magnetic data storage tape is generally allowed to move perpendicular or laterally to a direction of tape motion. The lateral motion of the tape maybe due to tape path tolerances and dimensional variations of the tape path built into the drive. Examples of tolerances that allow for lateral motion of the tape may include cartridge reel height, take-up reel height, guide heights, tape guide flange-to-flange spacing, take-up and supply reel flange-to-flange spacing, and tape width variations among several other tolerances. Further, as tape is generally read and written by the tape head in both directions the skew may vary with the direction of tape travel.
During data writing operations, separate channels allow for simultaneous read and write operations to a particular data track. Simultaneous read and write operations are know as read-while-write, or read-verity-write, where data is read immediately after being written to the tape to confirm the correct storage of data on the tape. Tape skew may limit the ability to read-while-write for a given data track and read/write element dimensions, since the read element may not be aligned with the data track written by the write elements. To compensate for tape skew, the data tracks are written with sufficient width, such that the read head will be on track during the maximum expected tape skew events. However, writing the tracks with sufficient width to compensate for tape skew, limits the possible density of data tracks for a given tape width and correspondingly limits the storage capacity of the tape. Accordingly, tape skew is one factor that limits the track density and data storage capacity of a magnetic data storage tape.
A system for limiting tape skew is disclosed in United States Patent Application Publication; Pub. No.: 2006/0103,968 A1, to Jurneke. Disclosed therein is a system for positioning a transducer head to a storage medium. The system purportedly compensates for skew of a storage medium, such as magnetic data storage tape and includes a transducer head assembly that includes read and write elements, at least one actuator for adjusting the azimuth position of the transducer head, first and second position sensors, and a controller. The first and second sensors sense a reference associated with a position of the tape, where the first and second sensors are positioned on opposite sides of a centerline of the read and write elements of the transducer head along a direction of tape movement. The sensed positions of the reference on opposite sides of the read and write elements may indicate the relative slope or skew of the tape and data tracks thereon to the transducer head.
The controller adjusts the azimuth position of the transducer head in response to sensed positions of the reference by the first and second sensors. Adjustments to the transducer head may be made on the fly during reading and writing operations. Further, the reference associated with the position of the tape may include one or more edges of the tape that may be a magnetically and/or optically detectable feature of the tape. The transducer head may be adjusted by differential actuators, such as piezoelectric actuators, which rotate the transducer head around its center of mass.
The system disclosed by Jurneke also includes a method for detecting the position of a transducer head with respect to a storage medium. The method includes sensing a reference associated with a position of a storage medium at a first position along a direction of storage medium transport, sensing the reference associated with the position of the storage medium at a second position along the direction of storage medium transport, wherein the first position and the second position are on opposite sides of a transducer head along a direction of storage medium transport, and positioning the azimuth of the transducer head relative to the storage medium in response to the sensed first position and the second position of the reference.
A disadvantage of the disclosed system is that adjusting the position of the transducer head about the center of mass of the head may not offer optimum alignment of the head with the linear data tracks. Ideally, the center of the transducer head should be coincident about a center of mass of the actuator assembly that adjusts the head, including any supporting structure and cabling.
Another known disadvantage of the disclosed system is that means used to adjust the azimuth and other positioning of the transducer head, such as stepper motors and gears, may not be able to dynamically adjust the head quickly enough to maintain proper track following. Further, stepper motor and gear systems are known to fail over time due to wear, for example.
A magnetic tape head tilting mechanism for tilting the magnetic head to a desired azimuth angle system for limiting tape skew is disclosed in U.S. Pat. No. 6,307,718, to Kasetty. Described therein is a magnetic tape head tilting mechanism that uses a worm gear and worm. The worm gear is driven by a stepper motor which adjusts the position of the magnetic head during recording of the signal onto the tape by the write head gap to maintain the strongest signal that is received by the read head gap. Fine angular resolution enables writing very high track densities and providing strong read signals. However, a disadvantage of the disclosed mechanism is that a worm gear and stepper motor system does not provide sufficient bandwidth or reliability needed to support high speed track following. Further, these systems are prone to wear and may fail prematurely.
U.S. Pat. No. 5,680,278, to Sawtelle Jr., describes a mechanism and method for providing rotational and linear movement to an azimuth tape recording head. The disclosed mechanism includes a toothed gear upon which the read/write head is mounted. Two identical threaded shafts, each extending from identical stepper motors are mounted on a base, along a same vertical plane as the toothed gear. Rotational axes of the threaded shads are arranged in a parallel relation such that each is meshably engaged to the toothed gear on opposing sides. Rotational movement of the head is achieved by synchronously rotating the threaded shafts, at the same speed, in opposite directions. Linear movement, or lifting/lowering, of the head is achieved by synchronously rotating one threaded shaft in one direction and the other threaded shaft in the opposite direction. As discussed above, a known disadvantage of worm gear and stepper motor systems is that they do not provide sufficient bandwidth or reliability needed to support high speed track following and that they are prone to wear.